Search

Agricultural management practices can induce changes in soil aggregation structure that alter the microbial nitrous oxide (N 2 O) production and reduction processes occurring at the microscale, leading to large-scale consequences for N 2 O emissions. However, the mechanistic understanding of how organic fertilization affects these context-dependent small-scale N 2 O emissions and associated key nitrogen (N) cycling microbial communities is lacking. Here, denitrification gas (N 2 O, N 2 ) and potential denitrification capacity N 2 O/(N 2 O + N 2 ) were assessed by automated gas chromatography in different soil aggregates (>2 mm, 2-0.25 and <0.25 mm), while associated microbial communities were assessed by sequencing and qPCR of N 2 O-producing (nirK and nirS) and reducing (nosZ clade I and II) genes. The results indicated that organic fertilization reduced N 2 O emissions by enhancing the conversion of N 2 O to N 2 in all aggregate sizes. Moreover, potential N 2 O production and reduction hotspots occurred in smaller soil aggregates, with the degree depending on organic fertilizer type and application rate. Further, significantly higher abundance and diversity of nosZ clades relative to nirK and nirS revealed complete denitrification promoted through selection of denitrifying communities at microscales favouring N 2 O reduction. Communities associated with high and low emission treatments form modules with specific sequence types which may be diagnostic of emission levels. Taken together, these findings suggest that organic fertilizers reduced N 2 O emissions through influencing soil factors and patterns of niche partitioning between N 2 O-producing and reducing communities within soil aggregates, and selection for communities that overall are more likely to consume than emit N 2 O. These findings are helpful in strengthening the ability to predict N 2 O emissions from agricultural soils under organic fertilization as well as contributing to the development of net-zero carbon strategies for sustainable agriculture., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 The Authors. Published by Elsevier B.V. All rights reserved.)

The denitrification process in aquaculture systems plays a crucial role in nitrogen (N) cycle and N budget estimation. Reliable models are needed to rapidly quantify denitrification rates and assess nitrogen losses. This study conducted a comparative analysis of denitrification rates in fish, crabs, and natural ponds in the Taihu region from March to November 2021, covering a complete aquaculture cycle. The results revealed that aquaculture ponds exhibited higher denitrification rates compared to natural ponds. Key variables influencing denitrification rates were Nitrate nitrogen (NO 3 -N), Suspended particles (SPS), and chlorophyll a (Chla). There was a significant positive correlation between SPS concentration and denitrification rates. However, we observed that the denitrification rate initially rose with increasing Chla concentration, followed by a subsequent decline. To develop parsimonious models for denitrification rates in aquaculture ponds, we constructed five different statistical models to predict denitrification rates, among which the improved quadratic polynomial regression model (SQPR) that incorporated the three key parameters accounted for 80.7% of the variability in denitrification rates. Additionally, a remote sensing model (RSM) utilizing SPS and Chla explained 43.8% of the variability. The RSM model is particularly valuable for rapid estimation in large regions where remote sensing data are the only available source. This study enhances the understanding of denitrification processes in aquaculture systems, introduces a new model for estimating denitrification in aquaculture ponds, and offers valuable insights for environmental management.- -N), Suspended particles (SPS), and chlorophyll a (Chla). There was a significant positive correlation between SPS concentration and denitrification rates. However, we observed that the denitrification rate initially rose with increasing Chla concentration, followed by a subsequent decline. To develop parsimonious models for denitrification rates in aquaculture ponds, we constructed five different statistical models to predict denitrification rates, among which the improved quadratic polynomial regression model (SQPR) that incorporated the three key parameters accounted for 80.7% of the variability in denitrification rates. Additionally, a remote sensing model (RSM) utilizing SPS and Chla explained 43.8% of the variability. The RSM model is particularly valuable for rapid estimation in large regions where remote sensing data are the only available source. This study enhances the understanding of denitrification processes in aquaculture systems, introduces a new model for estimating denitrification in aquaculture ponds, and offers valuable insights for environmental management., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Ltd. All rights reserved.)

Biochar-mediated change in soil N transformations has gained much attention. Biochar properties undergo changes through ageing process that may impact variably on N cycling, yet the degree to which soil N transformations may be impacted by biochar ageing process on long-time scale remains unclear. Here, we compared the effects on nitrification, denitrification and N 2 /N 2 O gas flux of short-term fresh biochar addition with long-term biochar amendment over six consecutive years in a paddy soil under identical amounts of soil organic C contents. We found that short-term treatments elicited a stronger effect on nitrification than long-term treatments, due to more positive effects on soil pH and ammonia monooxygenase gene (amoA) abundance of ammonia oxidizing bacterial (AOB). However, there were contrasting effects on soil denitrification and total N gas loss for two types of biochar addition, where the long-term treatments limited availability of C and N, and decreased net NO 3 - reduction and total N gas (N 2 O + N 2 ) losses by up to 55.5% and 25%. Short-term biochar addition increased dissolved organic C, nutrient content and soil pH, enhanced net NO 3 O emissions, but increased total N gas loss by 0.2-1.2 times. Our study quantified the differences in nitrification and denitrification capacity in response to short-term biochar addition and long-term biochar amendment and bridged the knowledge-gaps of fresh and aged biochar-mediated N transformation.- reduction by 88.7%, also reduced the ratio of nitrite reductase genes (nirS and nirK) and nitrous oxide reductase gene (nosZ) and N 2 O emissions, but increased total N gas loss by 0.2-1.2 times. Our study quantified the differences in nitrification and denitrification capacity in response to short-term biochar addition and long-term biochar amendment and bridged the knowledge-gaps of fresh and aged biochar-mediated N transformation., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

Effects of antibiotics on the transformation of nitrate and the associated N 2 O release in paddy fields are obscure. Using soil slurry experiments combined with 15 N tracer techniques, the influence of tetracycline and sulfamethazine (applied alone and in combination) on the denitrification, anaerobic ammonium oxidation (anammox), dissimilatory nitrate reduction to ammonium (DNRA) and N 2 O release rates in the paddy soil were investigated, while genes related to nitrate reduction and antibiotic resistance were quantified to explore the microbial mechanisms behind the antibiotics' effects. The potential rates of denitrification, anammox, and DNRA were significantly (p < 0.05) reduced, which were mainly attributed to the inhibitory effects of the antibiotics on nitrate-reducing microbes. However, the N 2 O release rates were significantly (p < 0.05) stimulated by the antibiotic treatments (0.6-6000 μg kg -1 soil dry weight), which were caused by the different inhibition effects of antibiotics on N 2 O production and N 2 O reduction as suggest by the changes in abundance of nirS (nitrite reduction step) and nosZ (N 2 O reduction to N 2 step) genes. Antibiotic resistance gene (tetA, tetG, sulI, and sulIII) abundances were significantly (p < 0.05) increased under high antibiotic exposure concentrations (>600 μg kg -1 soil dry weight). Our results suggest that the widespread occurrence of antibiotics in paddy soils may pose significant eco-environmental risks (nitrate accumulation and greenhouse effects) by altering nitrate transformation processes., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

Biochar is an efficacious amendment for mitigating nitrous oxide (N2O) emissions in soils. Nevertheless, the underlying mechanisms responsible for reduced N2O emissions by biochar in paddy soils remain inadequately elucidated. Here, using two typical paddy soils with contrasting pH values (5.40 and 7.56), the N2 and N2O fluxes and the associated functional genes were investigated in soil amended with varying amounts of biochar (0%, 0.5%, and 5%, weight/weight) via soil slurry incubation integrated with the N2/Ar technique and qPCR analysis. The results showed that N2O fluxes were significantly (p < 0.05) reduced by 0.65–3.64 times following biochar amendment, concomitant with a significant (p < 0.05) increase in N2 fluxes (5.47–46.14%) in both acidic and alkaline paddy soils. As a result, the N2O/(N2O + N2) ratios were significantly (p < 0.05) reduced by 1.53–4.65 fold in both soil types. In acidic paddy soils, the enhanced denitrification rates and the decreased N2O/(N2O + N2) ratios exhibited a strong correlation with increased pH values. In alkaline paddy soil, these changes were ascribed to the enhanced nosZ Clade I gene abundance and nosZ/(nirS + nirK) ratio. Our findings reveal that biochar primarily mitigates N2O emissions in paddy soils by promoting its reduction to N2. [ABSTRACT FROM AUTHOR]

Numerous interrelated factors (e.g., the labile C, soil NO3− concentration, and soil moisture content) are involved in controlling the microbial sources of N2O and the product stoichiometry of denitrification; however, the interactions among different factors are still poorly understood. Here, a fully robotized continuous flow soil incubation system (allowing simultaneous measurements of N2 and N2O fluxes) was employed to investigate the interactive effects of a 51-day duration of moist spell, straw amendment, and the NO3− level on the rate and product stoichiometry (N2O/(N2O + N2) ratio) of denitrification in heavily N loaded arable soils (i.e., paddy, vegetable, and orchard soils). The rewetting-induced N2O emissions mainly originated from bacterial denitrification in all soil types, with a clear shift to fungal denitrification (plus contingent nitrification) over time. The vegetable and orchard soils showed a higher share of bacterial N2O (62–70%) than that in the paddy soils (50–54%), which may be attributed to more labile-C driven bacterial activity induced by the greater manure and crop residue input therein. Interestingly, the inhibitory effect of high soil NO3− on N2O reduction in these soils was offset by a 51-day-long moist spell, regardless of the amendment of straw. To our knowledge, our study is the first to show that the inhibitory effect of high residual NO3− on N2O reduction is suppressed by a moist spell with a certain duration in heavily N loaded arable soils, suggesting that the water regime history should be considered when optimizing the N fertilizer application timing to mitigate soil N2O emissions. [ABSTRACT FROM AUTHOR]

Crop residue amendment in conjunction with synthetic nitrogen (N) fertilization is a common agricultural practice that increases soil fertility and crop yield. However, such a practice may also change soil denitrification process. Here, we conducted an incubation experiment with a robotized continuous flow N2 free incubation system [using helium (He) and oxygen (O2)] and measured fluxes of N2O and N2 at a high resolution in an intensively managed soil to examine the interaction effect of straw amendment and N fertilization on soil denitrification. Four treatments were set consisting of (a) a nonamended treatment (Control); (b) a Straw treatment (2 g straw kg−1 dry soil); (c) a KNO3 treatment (KNO3, 15 mM KNO3); and (d) a Straw + KNO3 treatment (2 g straw kg−1 dry soil and 15 mM KNO3). During the oxic phase (80% He plus 20% O2) of the experiment (initial 2 days), flux rates of N2O were 0.54 and 0.38 kg N/ha day−1 in the Control and KNO3 treatments, respectively. Meanwhile, straw amendment triggered N2O fluxes immediately after the onset of treatments, which was more evident in the Straw + KNO3 treatment. During the anoxic phase (100% He), both N2O and N2 emissions increased in all treatments, with the effect being more pronounced in the Straw and Straw + KNO3 treatments. In line with the observed differences in gas fluxes, the abundances of nirK, nirS, and nosZ genes increased clearly in these treatments. Overall, the mean N2O/(N2O + N2) ratio (0.69 ± 0.03) in the Control treatment was significantly lower compared to the KNO3 treatment (18.8%) and higher than the straw‐amended soils (31.9% and 17.4% compared to the Straw and Straw + KNO3 treatments, respectively). Taken together, our results suggest straw amendment significantly altered N2O and N2 fluxes by decreasing the N2O/(N2O + N2) ratio; however, the effects of straw amendment depended on soil nitrate content. [ABSTRACT FROM AUTHOR]

Current estimates of global indirect N 2 O emissions are based on a relatively small dataset and remain a major source of uncertainly in the global N 2 O budget. Nitrogen (N)-enriched groundwater from agricultural fields may act as an important source of indirect N 2 O emissions as it discharges to adjacent watershed areas. During 2015–2017, dissolved N 2 O concentrations in groundwater were measured and indirect N 2 O emission factors (EF 5g) calculated under three typical high-N land-use types (vineyard, vegetable field and paddy field) in eastern China. The average dissolved N 2 O concentrations in groundwater were 58.1 ± 40.4, 18.5 ± 11.5 and 0.72 ± 0.27 μg N L−1 for vineyard, vegetable field and paddy field, respectively. The dissolved N 2 O was over-saturated and was therefore a net source of N 2 O to the atmosphere. The indirect N 2 O emission factors (EF 5g) of vineyard (0.0091) and vegetable (0.0092) fields were much higher than the current Intergovernmental Panel on Climate Change (IPCC) default value of 0.0025 which indicated that these land-uses may have led to indirect N 2 O emissions from the underlying groundwater. In contrast, the EF 5g of the paddy field (0.0019) was slightly lower than the default EF 5g proposed by IPCC (2006) and contributed minimal indirect N 2 O emissions to the atmosphere. However, the current IPCC method may have overestimated the contribution of groundwater N 2 O to the global N cycle because it took residual but not initial groundwater NO 3 −-N concentration into account when calculating EF 5g. Therefore, we proposed the adoption of an improved method for calculating the EF 5g and compared it to the current IPCC (2006) method using data from the present study and other published data. The results of the comparison showed that the improved method was more scientifically appropriate measurement for calculating EF 5g. Image 1 • Vineyard and vegetable field is important sources of indirect N 2 O emission. • Paddy soil contributed minimal indirect N 2 O emission to the atmosphere. • Estimates of EF 5g should include denitrified NO 3 −-N in groundwater. Vegetable field and vineyard can bring about considerable indirect N 2 O emissions from the underlying groundwater to the atmosphere and the NO 3 −-N consumed by denitrification should take into account in estimating EF 5g. [ABSTRACT FROM AUTHOR]

Effects of antibiotics on the transformation of nitrate and the associated N 2 O release in paddy fields are obscure. Using soil slurry experiments combined with 15 N tracer techniques, the influence of tetracycline and sulfamethazine (applied alone and in combination) on the denitrification, anaerobic ammonium oxidation (anammox), dissimilatory nitrate reduction to ammonium (DNRA) and N 2 O release rates in the paddy soil were investigated, while genes related to nitrate reduction and antibiotic resistance were quantified to explore the microbial mechanisms behind the antibiotics’ effects. The potential rates of denitrification, anammox, and DNRA were significantly ( p  < 0.05) reduced, which were mainly attributed to the inhibitory effects of the antibiotics on nitrate-reducing microbes. However, the N 2 O release rates were significantly ( p  < 0.05) stimulated by the antibiotic treatments (0.6–6000 μg kg −1 soil dry weight), which were caused by the different inhibition effects of antibiotics on N 2 O production and N 2 O reduction as suggest by the changes in abundance of nirS (nitrite reduction step) and nosZ (N 2 O reduction to N 2 step) genes. Antibiotic resistance gene ( tetA , tetG , sulI, and sulIII ) abundances were significantly ( p  < 0.05) increased under high antibiotic exposure concentrations (>600 μg kg −1 soil dry weight). Our results suggest that the widespread occurrence of antibiotics in paddy soils may pose significant eco-environmental risks (nitrate accumulation and greenhouse effects) by altering nitrate transformation processes. [ABSTRACT FROM AUTHOR]

Denitrification is one of the most challenging nitrogen (N)‐cycling processes to quantify due to complex interactions among influencing factors. In shallow water‐sediment systems, ubiquitous periphytic biofilms consisting of a consortium of algae, bacteria, and nutrients account for additional interactions. The challenge in improving denitrification models is to determine how each factor directly affects denitrification and how those factors indirectly impact denitrification via interactions with other factors. We combined incubation experiments and structural equation modeling to reveal the interaction networks of biotic and abiotic factors and their roles in net denitrification. The N species in the sediment exerted much greater control over net denitrification than that exerted by the N species in the water column. In addition, the relative abundances of denitrifier and nitrifier genes in the sediment showed a dominant control over net denitrification that was similar to the control by N species but was affected by abiotic factors. Periphytic biofilm increased net denitrification both directly and indirectly by altering the pH and the dissolved oxygen, dissolved organic carbon, and N concentrations in the water column and sediment. The overall contribution of periphytic biofilm to net denitrification was less than that of the sediment but greater than that of the water column. Our findings highlight the importance of incorporating bacterial denitrifier and nitrifier genes and periphytic biofilm characteristics during model construction when predicting denitrification in a shallow and stagnant water‐sediment system. Key Points: SEM was used to reveal the interaction networks of biotic and abiotic factors and their roles in net denitrificationThe periphytic biofilm increased the denitrification rate directly and indirectlyWe suggest improving model precision by including periphytic and bacterial factors [ABSTRACT FROM AUTHOR]

Previous studies of long-term manure applications in paddy soil mostly focused on the effects on denitrification, occurrence of antibiotics and antibiotic resistance genes (ARGs) without considering the effects on anaerobic ammonium oxidation (anammox). Here, we investigated the potential rates of anammox and denitrification, occurrence of antibiotics and AGRs in response to three fertilization regimes (C, no fertilizer; N, mineral fertilizer; and NM, N plus pig manure) in six long-term paddy experiment sites across China. The potential rates of anammox (0.11–3.64 nmol N g −1 h −1 ) and denitrification (1.5–29.05 nmol N g −1 h −1 ) were correlated with the abundance of anammox genes ( hzsB ) and denitrification functional genes ( narG, nirK, nirS and nosZ ), respectively. The anammox and denitrification rates were affected by soil organic carbon (SOC) and significantly ( p  < 0.05) increased in NM treatments relative to those in N treatments. Although pig manure application increased antibiotic concentrations and abundance of ARGs compared with N treatments, the increased antibiotics did not directly affect the anammox and denitrification rates. Our results suggested that long-term pig manure application significantly increased antibiotic concentrations, abundance of ARGs, and rates of anammox and denitrification, and that the effects of pig manure-derived antibiotics on anammox and denitrification were marginal. This is the first report that investigates the effects of long-term pig manure application on anammox in paddy soils. More attention should be paid to the potential ecological risk of increased ARGs caused by pig manure application in paddy soils. [ABSTRACT FROM AUTHOR]

Anaerobic ammonium oxidation (anammox process) widely occurs in paddy soil and may substantially contribute to permanent N removal; however, little is known about the factors controlling this process. Here, effects of temperature, pH, organic C, and substrates on potential rate of anammox and the relative contribution of anammox to total N2 production in a paddy soil were investigated via slurry incubation combined with 15N tracer technique. Anammox occurred over a temperature range from 5 to 35 °C with an optimum rate at 25 °C (1.7 nmol N g−1 h−1) and a pH range from 4.8 to 10.1 with an optimum rate at pH 7.3 (1.7 nmol N g−1 h−1). The presence of glucose and acetate (5-100 mg C L−1) significantly inhibited anammox activities and the ratio of anammox to total N2 production. The response of potential rates of anammox to ammonium concentrations fitted well with Michaelis-Menten relationship showing a maximum rate (Vmax) of 4.4 nmol N g−1 h−1 and an affinity constant (Km) of 6.3 mg NH4+-N L−1. Whereas, nitrate addition (5-15 mg 15NO3−-N L−1) significantly inhibited anammox activities and the ratio of anammox to total N2 production. Our results provide useful information on factors controlling anammox process and its contribution to N loss in the paddy soil. [ABSTRACT FROM AUTHOR]

The adverse effects of increased nitrate (NO 3 −) pollution especially from the non-point source on the hydrosphere and anthroposphere are becoming more prominent. The non-point-derived NO 3 − in the rivers supplying the upstream threatens the aquatic ecosystem of Taihu Lake. Here, dual-stable isotopes (δ15N and δ18O) of NO 3 − were applied to the Bayesian model (SIAR) for quantitative source identification of reactive nitrogen (Nr) in a mixed agricultural and urban region along the complex river network of the Yangze River delta. The results showed that the NO 3 − concentrations in the rivers ranged from 1.09 to 4.44 mg L−1 and decreased from the highly urbanized areas to the lakeside rural areas. The specific isotopic characteristics of four sources (atmospheric deposition, AD; chemical fertilizer, CF; manure and sewage, MS; and soil leachate, SL) by the SIAR isotope model indicated that the MS source made the greatest contribution (46.56%) to the total NO 3 − load, followed by SL (27.86%), CF (23.77%), and AD (1.81%). The highly urbanized areas and the hybrid areas, which contained a mix of industrialized, populated, and agricultural areas, were identified as hotspot areas with heavy Nr pollution, responsible for spatial patterns of δ15N–NO 3 - and δ18O–NO 3 -. These hotspot areas were characterized by a less well-developed sewage pipeline system with high Nr emissions from cash crops. The changes in wastewater treatment level, the agricultural production structure, and meteorological changes were the main factors of spatial variation of Nr concentration and source in the upstream Taihu Lake Basin. The variation in Nr concentration across Taihu Lake would respond to these anthropogenic-driven Nr loads. These findings suggest that MS was the predominant source had the strongest effect on the overall riverine NO 3 − source which was the primary problem that needed to be solved. [Display omitted] • Severe nitrate contamination was found in Yangtze Delta river network areas. • Dual isotopes and SIAR model were applied to reveal urban-rural nitrate sources. • Manure and sewage dominated nitrate source, accounting for 37.35%–47.85%. • UBA and IDA were anthropogenic-driven heavily polluted hot spot areas. [ABSTRACT FROM AUTHOR]

Knowledge of denitrification process and its end-product at various depths of paddy soil is very important for our understanding of its role in mitigating reactive N and indirect nitrous oxide (N 2 O) emission. In this study, the end-products of denitrification were determined at four depths in a long-term field lysimeter experiment in southeast China over a rice season. Three treatments were included: (1) chemical fertilizer (NPK); (2) NPK plus pig manure (NPKM); and (3) NPK plus straw (NPKS). The concentration of dissolved N 2 O increased with soil depth across all treatments and the highest concentration of excess dinitrogen (N 2 ) was observed at 0.2 m depth, as was the highest dissolved organic carbon (DOC) content. Denitrification reduced the amount of nitrate by 48–54% in the paddy soil profile, especially at 0.2 m depth (68–88%), whereas the lower reduction of NO 3 − (17–44%) in the subsoil (at 0.6 and 0.8 m depth) was accompanied by a higher concentration of NO 3 − . Our results demonstrated that DOC was the major limiting factor of denitrification in the subsoil. The application of pig manure markedly increased the amount of DOC in the surface soil, resulting in a high rate of denitrification, whereas the addition of straw had no effect on denitrification. The indirect emission factors for N 2 O (EF5-g, 0.001–0.006) were comparable with the default value (0.0025) reported by the Intergovernmental Panel on Climate Change . The low N 2 O production was probably caused by the complete reduction of N 2 O to N 2 , as reflected by the lower N 2 O/(N 2 O + N 2 ) ratios in the paddy soil profile. Although the surface soil was identified as a hotspot for denitrification, a considerable amount of excess N 2 was observed in the subsoil for all three treatments. We therefore conclude that the loss of N through denitrification may be significantly underestimated if only the surface soil is considered. [ABSTRACT FROM AUTHOR]

The effects of microplastics (MPs) from agricultural plastic films on soil nitrogen transformation, especially denitrification, are still obscure. Here, using a robotized flow-through system, we incubated vegetable upland soil cores for 66 days with MPs from PE mulching film (F-PE) and PVC greenhouse film (F-PVC) and directly quantified the emissions of nitrogenous gases from denitrification under oxic conditions, as well as the denitrification potential under anoxic conditions. The impact of MPs on soil nitrogen transformation was largely determined by the concentration of the additive phthalate esters (PAEs) containing in the MPs. The F-PE MPs with low level of PAEs (about 0.006 %) had no significant effect on soil mineral nitrogen content and nitrogenous gas emissions under oxic conditions. In contrast, the F-PVC MPs with high levels of PAEs (about 11 %) reduced soil nitrate content under oxic conditions, probably owing to promoted microbial assimilation of nitrogen, as the emissions of denitrification products (N 2 , NO, and N 2 O) was not affected. However, the F-PVC MPs significantly enhanced the denitrification potential of the soil due to the increased abundance of denitrifiers under anoxic conditions. These findings highlight the disturbance of MPs from agricultural films, particularly the additive PAEs on nitrogen transformation in soil ecosystems. [Display omitted] • MPs of pure PVC and PE did not affect nitrogenous gas emission in the tested soil. • MPs from PVC greenhouse film (F-PVC) with high PAEs reduced soil NO3− content. • F-PVC increased CO 2 emission but did not alter nitrogenous gas emissions. • F-PVC increased soil denitrification potential under anoxic conditions. • F-PVC altered soil bacterial community and enriched PAEs-degrading bacteria. [ABSTRACT FROM AUTHOR]

Organic fertilizer can enhance soil health and multifunctionality in agroecosystems, but its impact on soil-borne greenhouse gas emissions needs mitigation. Fungal denitrification significantly contributes to N 2 O emissions in carbon-rich soils; yet, the interactions between bacterial and fungal denitrifers under organic fertilizer amendment, remain unclear. Here, we investigated the rates and proportions of N 2 O and N 2 emissions, along with the interactions between fungal and bacterial denitrifiers in a high nitrogen (N) loading arable soil subjected to four treatments: ⅰ) Control, ⅱ) organic fertilizer (Manure), ⅲ) synthetic fertilizer (Urea), and ⅳ) synthetic plus organic fertilizer (Urea + Manure). Results showed that N 2 O and N 2 fluxes increased by 35.4 and 7.7 folds, respectively, in the Manure treatment compared to Control treatment. And these fluxes increased by 62.9 and 37.0 folds, respectively, in the Manure + Urea treatment compared to Urea treatment. Meanwhile, the contribution of fungal denitrification to N 2 O emissions significantly increased in both Manure and Urea + Manure treatments, due to the significant enrichment of keystone fungal denitrifiers like Chaetomium among bacterial and fungal denitrifiers' co-occurrence networks. Additionally, N 2 O/(N 2 O + N 2) ratio significantly decreased in the Manure and Urea + Manure treatments, which was primarily driven by significant enrichment of keystone bacterial denitrifiers carrying nosZ gene such as Achromobacter , Chelatococcus , and Shinella. These bacteria possess complete denitrification capability and can synergize with fungal denitrifiers, enhancing N 2 O reduction. Overall, our findings suggest that organic fertilizer amendment in high N loading arable soils decreases N 2 O/(N 2 O + N 2) ratio mainly by enhancing fungal-bacterial denitrifier mutualism. Conceptual diagram illustrating how organic fertilizer amendment regulates N 2 O and N 2 emissions in high nitrogen loading arable soils by changing the abundance, community structure, and co-occurrence pattern of bacterial and fungal denitrifiers. [Display omitted] • Organic fertilizer increased N 2 O and N 2 emissions while decreasing N 2 O/(N 2 O + N 2) ratio. • Organic fertilizer significantly increased the share of soil fungal denitrification. • Organic fertilizer enriched bacterial denitrifiers harboring nosZ Clade Ⅰ gene. • Organic fertilizer enhanced the mutualism between bacterial and fungal denitrifiers. [ABSTRACT FROM AUTHOR]

[Display omitted] • Denitrification rates initially increase and subsequently decline with the increased water flow velocities, regardless of the presence of vegetation. • Denitrification rates are significantly higher at night than daytime with vegetated sediment. • Flow velocity regulates denitrification by balancing the impact on NO 3 − transfer and dissolved oxygen saturation. Ditches and river networks play crucial roles in nitrogen (N) removal within aquatic ecosystems, and their effectiveness depends on various biogeochemical and hydraulic factors. Among these, water flow velocity, as a hydraulic factor, has been relatively underexplored due to the limited N removal quantifying method in flowing open channel waterway. In this study, we used argon as a tracer to calibrate the diffusion coefficient of N 2. We then applied the gas diffusion coefficient method to assess how water flow velocity affects denitrification rates. Our results provide new insights into the trade-off role of water flow velocity in N removal through denitrification across a range of flow velocities (0, 1, 4, 6, 10 cm s−1) in open channel waterway. This trade-off effect arises from the interplay between increased NO 3 − transfer and rising dissolved oxygen levels as water flow velocity increases, resulting in an initially increased and subsequently declined denitrification rate with increased water flow velocities. The denitrification rates reached maximum and minimum values at the water flow velocity of 4 cm s−1 and 0 cm s−1, respectively, regardless of the presence of vegetation. Average denitrification rates ranged from 36.8 to 358.8 µmol N 2 m−2h−1 in unvegetated sediments and from 133.0 to 383.5 µmol N 2 m−2h−1 in vegetated sediments within the spectrum of water flow velocities. In addition, we observed significantly higher denitrification rates at night than during the day, likely due to the photosynthesis and respiration processes associated with plant metabolism. These findings of this study contribute to the understanding of how water flow velocity affects denitrification rates and provide valuable information for the simulation and restoration efforts aimed at enhancing the N removal capacity of river networks. [ABSTRACT FROM AUTHOR]

Highlights • Gross mineralization and immobilization rates were greater in forest than in agricultural soils. • Gross nitrification rates were lower in forest than in agricultural soils. • Gross mineralization and immobilization rates were tightly coupled in forest soils. • Nitrate production and leaching risk were higher in agricultural than in forest soils. Abstract A better understanding of gross nitrogen (N) transformation rates in agroforestry systems is crucial for testing the land use management effects on soil N retention. In this study, gross N transformation rates in forest and adjacent agricultural soils in two agroforestry systems, hedgerow and shelterbelt, located near Edmonton, Alberta, Canada were investigated using the 15N labeled technique and a numerical model, FLUAZ. Gross N mineralization rates in the forested land use were higher than that in the agricultural land use for hedgerow and shelterbelt agroforestry system, and the pattern was the same for gross N immobilization rates. However, gross nitrification rates in the forested land use were lower than that in the agricultural land use for both agroforestry systems. The ratios of gross nitrification to ammonium immobilization rates and the ratios of gross N mineralization to immobilization rates in the agricultural land use were higher than that in the forested land use irrespective of the type of agroforestry system studied. Our study suggested that the agricultural land use had a lower potential to conserve N, posing a higher risk of N loss, whereas mineralization and immobilization processes were tightly coupled in the forested land use, leaving less N available for nitrification and subsequent N loss to the environment. [ABSTRACT FROM AUTHOR]

Abstract Straw application in combination with synthetic N fertilizer could increase crop yield and improve soil fertility, however, contradictory observations have been reported on the effects of straw addition on soil N 2 O emission. Straw application can affect both denitrification rate and its product stoichiometry (N 2 O/(N 2 O + N 2) ratio), whereas the latter remains rather unclear since the ratio is strongly regulated by other soil parameters, e.g. nitrate and oxygen concentrations at denitrifying micro-sites. In this context, we conducted an incubation experiment with a robotized continuous flow incubation system using a He/O 2 atmosphere and measured N 2 O and direct N 2 fluxes over 22 days. Soil amended with and without rice straw (2.5 g kg−1 soil) in conjunction with nitrate fertilizer (10 mM KNO 3) and non-amended control soil were incubated under 85% water-filled pore space. To simulate a short soil anoxic period, three different O 2 partial pressures phases were set (20%, 5% and 10%). Additionally, N 2 O site preference signatures of soil-emitted N 2 O were analyzed to identify the processes contributing to N 2 O fluxes. Addition of nitrate increased cumulative N 2 O fluxes and decreased cumulative N 2 fluxes compared with non-fertilized control, while rice straw amendment increased both N 2 O and N 2 emissions drastically compared with the nitrate only treatment. The N 2 O SP values ranged from 0.4 to 2.7‰ among all treatments, indicating denitrification/nitrifier denitrification was the dominating source. The results suggest that straw amendment can trigger high denitrification rate, whereas the effect of straw amendment on the amount of emitted N 2 O and the N 2 O/(N 2 O + N 2) product ratio strongly depends on soil NO 3 − concentration. As a conclusion, the present study suggests that straw amendment in conjunction with nitrate-N can increase soil N 2 O emissions under conditions favoring denitrification, even though it may decrease the overall N 2 O/(N 2 O + N 2) product ratio. Highlights • Combined effect of straw addition and soil NO 3 − was shown on direct N 2 fluxes. • Decrease in soil NO 3 − caused a quick shift from N 2 O production to reduction. • Effect of straw addition on N 2 O and N 2 fluxes depends on soil NO 3 − content. [ABSTRACT FROM AUTHOR]

Terrestrial soil denitrification is of great importance for closing the nitrogen (N) cycle, yet the current understanding of soil denitrification response to N fertilization remains uncertain. While there has been a focus on factors controlling N 2 O fluxes from agricultural soils because of its global warming effect, much less is known about factors controlling total denitrification losses, yet these can be sufficiently large to affect N use efficiency. Here, we collated 353 observations from 74 papers and conducted a global-scale meta-analysis to explore the effects of N fertilization on agricultural soil denitrification (N 2 O + N 2 ) where the acetylene inhibition technique was used. Relative to the control, N fertilization significantly increased soil denitrification by an average of 174%, although the magnitude of this increase differed significantly across environmental and soil conditions. Soil denitrification was more responsive to N fertilization in grasslands than in croplands. The changes in soil denitrification increased exponentially when the rates of synthetic N fertilizer application ≤ 250 kg N ha − 1 , but above this threshold, there were no further increases. The responses of soil denitrification to N fertilization were negatively correlated with soil clay content, C:N ratio, and bulk density. The comparable responses of soil N 2 O emissions (165%) and denitrification to N fertilization resulted in a small insignificant decrease of the N 2 O:N 2 ratio. Organic fertilizer applied with and without synthetic N fertilizer can contribute to lower N 2 O emissions probably by facilitating the last step of soil denitrification to N 2 production. Taken together, we conclude that these findings can provide important insights on regulating soil denitrification, which might contribute to improvement of N use efficiency and elimination of its negative impacts in agro-ecosystems. [ABSTRACT FROM AUTHOR]

Biochar and nitrification inhibitors are increasingly being proposed as amendments to improve nitrogen use efficiency (NUE). However, their effects on soil denitrification and the major N loss in rice paddies over an entire rice-growing season are not well understood. In this study, using intact soil core incubation combined with N 2 /Ar technique, the impacts of biochar and a nitrification inhibitor (Ni), 2-chloro-6-(trichloromethyl)-pyridine, on rice yield and soil denitrification, as well as ammonia (NH 3 ) volatilization, were investigated over two rice-growing seasons in the Taihu Lake region of China. Field experiments were designed with four treatments: N0 (no N applied), N270 (270 kg N ha − 1 applied), N270 + C (25 t ha − 1 biochar applied) and N270 + Ni (2-chloro-6- [trichloromethyl] -pyridine, 1.35 kg ha − 1 N applied). Compared with single application of N fertilizer alone (N270), biochar (N270 + C) and Ni (N270 + Ni) applications increased rice yields by 4.2–5.2% and 6.2–7.3%, respectively. The cumulative N 2 -N and NH 3 -N losses in different treatments varied from 11.9 to 21.8% and from 11.5 to 22.0% of the applied N, respectively. Compared with the single application of N fertilizer, the Ni application increased total NH 3 emission by 4.0–20.6% and significantly decreased total N 2 -N emission by 9.7–19.4% (p < 0.05), while the biochar application increased total NH 3 and N 2 -N emissions by 8.6–17.9% and 3.3–9.7%, respectively. Overall, the biochar application resulted in an 11–15% higher net gaseous N than the Ni application. Although the biochar application may increase the rice yield and consequently the plant N uptake, it also promoted N loss more than Ni. Therefore biochar may not be good for maintaining soil fertility over a long period. Instead, applying Ni may be an optimal practice to ensure food security, while decreasing gaseous N loss, for rice production in the Taihu Lake region of China. [ABSTRACT FROM AUTHOR]

In China, the rapid conversion of traditional rice-wheat rotation to orchard-induced management practices has resulted in changes in soil N 2 O and NO emissions. However, quantification of emissions from orchard fields characterized by a high rate of organic and chemical N fertilization application has not yet been widely carried out. In this study, we measured soil N 2 O and NO emissions from a typical peach orchard in the Taihu region over a 2-year period using a combination of static chamber and gas chromatography techniques. Four treatments were examined: organic manure alone (OM), chemical fertilizers alone (CF), organic manure plus chemical fertilizers (OF + CF), and no fertilizers as a control (CK). No significant effects of organic amendments × chemical fertilizer application on soil N 2 O and NO emissions were observed. The mean annual N 2 O emission over the 2-year period was 3.2 kg N ha −1 under CK treatment, and increased to 9.3, 10.3, and 20.1 kg N ha −1 under OF, CF and OF + CF treatment, respectively. In contrast, cumulative NO emissions were relatively low, ranging from 0.14 to 1.99 kg N ha −1 yr −1 . The NO emissions were enhanced by approximately eight fold following mineral fertilization application either alone or in combination with organic manure, and to a lesser extent (45%, nonsignificant) by organic manure application alone. Overall, NO accounted for approximately 14.5 and only 2.9% of the total (NO + N 2 O)-N emissions under CF and OF treatment, respectively. Over the 2-year observation period, the N 2 O emission factor averaged 1.69%, 1.32% and 1.86% and the NO emission factor averaged 0.03%, 0.27% and 0.17% under OF, CF and OF + CF treatment, respectively. Overall, our results suggest that fertilized peach orchard soil in the Taihu region is an important source of N 2 O emissions. [ABSTRACT FROM AUTHOR]

Information about effects of conversion from rice fields to vegetable fields on denitrification process is still limited. In this study, denitrification rate and product ratio (i.e., N 2 O/(N 2 O + N 2) ratio) were investigated by soil-core incubation based N 2 /Ar technique in one rice paddy field (RP) and two vegetable fields (VF4 and VF7, 4 and 7 years vegetable cultivating after conversion from rice fields, respectively). Genes related to denitrification and bacterial community composition were quantified to investigate the microbial mechanisms behind the effects of land-use conversion. The results showed that conversion of rice fields to vegetable fields did not significantly change denitrification rate although the abundance of denitrification related genes was significantly reduced by 79.22%–99.84% in the vegetable soils. Whereas, compared with the RP soil, N 2 O emission rate was significantly (P < 0.05) increased by 53.5 and 1.6 times in the VF4 and VF7 soils, respectively. Correspondingly, the N 2 O/(N 2 O + N 2) ratio increased from 0.18% (RP soil) to 5.65% and 0.65% in the VF4 and VF7 soils, respectively. These changes were mainly attributed to the lower pH, higher nitrate content, and the altered bacterial community composition in the vegetable soils. Overall, our results showed that conversion of rice fields to vegetable fields increased the N 2 O emission rate and altered the product ratio of denitrification. This may increase the contribution of land-use conversion to global warming and stratospheric ozone depletion. [ABSTRACT FROM AUTHOR]

• Indirect N 2 O emissions were measured in a typic agricultural-urban gradient river. • Suburban agriculture increased river N loading but decreased indirect N 2 O emissions. • High DOC reduced the denitrification source N 2 O by favoring complete denitrification. • DOC factor can improve our EF 5r prediction in the agricultural-urban gradient river. The effects of land use on riverine N 2 O emissions are not well understood, especially in suburban zones between urban and rural with distinct anthropogenic perturbations. Here, we investigated in situ riverine N 2 O emissions among suburban, urban, and rural sections of a typical agricultural-urban gradient river, the Qinhuai River of Southeastern China from June 2010 to September 2012. Our results showed that suburban agriculture greatly increased riverine N concentration compared to traditional agricultural rivers (TAR). The mean total dissolved nitrogen (TDN) concentration was 8.18 mg N L−1 in the suburban agricultural rivers (SUAR), which was almost the same as that in the urban rivers (UR, of 8.50 mg N L−1), compared to that in TAR (0.92 mg N L−1). However, the annual average indirect N 2 O flux from the SUAR was only 27.15 μg N 2 O-N m−2 h−1, which was slightly higher than that from the TAR (13.14 μg N 2 O-N m−2 h−1) but much lower than that from the UR (131.10 μg N 2 O-N m−2 h−1). Moreover, the average N 2 O emission factor (EF 5r , N 2 O-N/DIN-N) in the SUAR (0.0002) was significantly lower than those in the TAR (0.0028) and UR (0.0004). The limited indirect N 2 O fluxes from the SUAR are best explained by the high riverine dissolved organic carbon (DOC) and low dissolved oxygen, which probably reduced the denitrification source N 2 O by favoring complete denitrification to produce N 2 and inhibited the nitrification source N 2 O, respectively. An exponential decrease model incorporating dissolved inorganic nitrogen and DOC could greatly improve our EF 5r predictions in the agricultural-urban gradient river. Given the unprecedented suburban agriculture in the world, more studies in suburban agricultural rivers are needed to further refine the EF 5r and better reveal the mechanisms behind indirect N 2 O emissions as influenced by suburban agriculture. [Display omitted] [ABSTRACT FROM AUTHOR]

Highlights: [•] We measured 29N2 and 30N2 using membrane inlet mass spectrometry. [•] Potential anammox rates varied spatially and temporally in the studied rivers. [•] Water temperature and in sediments were the main controls of anammox activity. [•] Candidatus Kuenenia was the dominated anammox species in the river sediments. [ABSTRACT FROM AUTHOR]

[Display omitted] • Soil gaseous N emissions were altered by land use conversion. • Land use conversion increased N 2 O emissions from bacterial denitrification. • Land use conversion altered the abundance and composition of soil denitrifiers. • Increased moisture in upland soils induced more gaseous N losses dominated by N 2 O. In this study, soil-borne N 2 , NO, and N 2 O emissions induced by land use conversion and water management were investigated in intact soil cores under a helium/oxygen atmosphere by a robotized incubation system in combination with the N 2 O 15N site preference signature and molecular-based microbial analysis. The experiment consisted of five treatments: i) paddy-flooded (PF); ii) orchard-wet (OW, 70% WFPS); iii) orchard-dry (OD, 43% WFPS); iv) vegetable-wet (VW, 70% WFPS); and v) vegetable-dry (VD, 43% WFPS). The vessels of each treatment received 200 mg urea-nitrogen (N) (equivalent to 210 kg of urea-N ha−1), and soil moisture in the OW and VW treatments was adjusted to a higher constant moisture to simulate a scenario after irrigation or rainfall. The results showed that total gaseous N losses during the incubation period were 25.33 ± 0.33 kg N ha−1 in the PF treatment, whereas smaller losses were recorded in the OD and VD treatments (4.28 ± 2.04 and 9.75 ± 3.75 kg N ha−1, respectively). The potential contribution of bacterial denitrification to N 2 O emissions in the OD and VD treatments was 11.1% and 15.4% higher, respectively, than that in the PF treatment (58.8% ± 0.5%). Furthermore, the corresponding N 2 O/(N 2 O + N 2) ratio in the OD and VD treatments decreased by 50% and 73.8%, respectively, relative to the ratio in the PF treatment (0.42 ± 0.01). Such changes indicated the crucial role of altered soil properties caused by land use conversion in regulating the production and consumption of N 2 O. Relative to the normal moisture (dry) condition, enhanced soil moisture increased total gaseous N losses in the orchard and vegetable soils by 386.9% and 67.4%, accompanied by a higher N 2 O/(N 2 O + N 2) product ratio, but decreased the share of bacterial N 2 O by 11.1% and 15.4%, respectively. The changes in the abundance and community composition of soil denitrifiers caused by land use conversion from rice paddies to orchards and vegetable fields could partly explain the differences in gaseous N loss therein. These findings highlight the influence of land use conversion on soil gaseous N emissions and demonstrate that increased moisture in upland soils reduced the dominance of bacterial denitrification in N 2 O production. [ABSTRACT FROM AUTHOR]

The amount of indirect nitrous oxide (N2O) emission from agricultural watersheds with high intensive fertilizer nitrogen (N) application rate is supposed to be great. However, limited data restrict the understanding of N2O emission in these areas, especially in water-rich watershed dominated by rice paddies. Indirect N2O emission and its potential drivers were studied for two years in the surface water of a rice paddy-dominated agricultural watershed in eastern China. Results showed that nitrate concentration (mean 0.4 mg N L−1) and Eh (mean of −86.1 mV) in surface water were the most important drivers of indirect N2O emission. The N2O emission rates of rivers (mean = 12.9 ± SD 21.8 μg N2O–N m−2 h−1) were significantly higher than those of ponds (mean = 4.5 ± SD 16.3 μg N2O–N m−2 h−1) and the reservoir (mean = 7.9 ± SD 10.0 μg N2O–N m−2 h−1). The indirect N2O emission only accounted for 1.2% of the total indirect and direct N2O emissions and 0.53% of N removed via aquatic denitrification. The disproportionately low N2O emissions could have resulted from the limited inputs of N into waterways and low N2O/(N2O + N2) in removing N through denitrification under strong reductive conditions. We suggest that the N2O budget predictive modeling should consider water Eh because it may indirectly affect N2O emission rates by controlling the ratio of N2O to N2 via denitrification. [ABSTRACT FROM AUTHOR]